Current art offers a great number of devices for monitoring a patient's vital signs. These monitors are regularly used in many hospitals, primarily in intensive care units. Thanks to modern technologies a patient doesn't have to be connected to a device by any cables and there is no longer a necessity of a patient and hospital personnel to cooperate for the successful monitoring of the patient's vital signs. These contactless monitoring systems can also be used in aftercare departments or nursing homes.
Most of these modern, but no longer overly unique, devices are based on a similar principle. The main component of the measuring device is a mat with one or more integrated sensors. These sensors may be of different types. One of the types is a sensor which senses changes in forces acting on the mat. Another method uses accelerometers measuring the vibrations of the mattress platform in order to measure vital signs. It is also possible to use, for example, piezoelectric sensors and also additional capacitive sensors. The mat is located under the location where the patient is recumbent generally under the mattress.
Many embodiments of mats are known, i.e. in patent application WO 2010080794 the mat is filled with a fluid and a pressure sensor senses changes in pressure caused by the patient's breath and pulse rate. The problem of the solution is the complexity of manufacturing a special mat filled with fluid.
One interesting solution is an evaluation of vital signs on the basis of an analysis of video signal. Some vital signs are calculated on the basis of the ratio of the intensity of light of two different wavelengths reflected from the patient's skin. Such a solution is described for example in patent application WO 2013027027. But this method is not very accurate and it is also difficult to make a measurement using this method under poor lighting conditions.
There are also known solutions where the measuring member is integrated in the mattress. We can see such a solution for example in U.S. Pat. No. 7,652,581. One disadvantage may be the price of the mattress adapted for this purpose.
Strain gauges, the main role of which is evaluation of a patient's weight, may also be used for the implementation of a measuring device. If they are correctly adapted they can also record the vibrations caused by breathing and heartbeat. But highly sensitive strain gauges are necessary for this method of measurement and they may be prone to interference and can often react to ambient forces which are not a subject of interest. We can see such a solution in U.S. Pat. No. 7,699,784.
As a result of the drop in the purchasing price of piezoelectric sensors they are used in many branches, from medical devices, uses in the army or for building security. Piezoelectric sensors are used in medical devices, for example in plethysmography, measuring of blood pressure, measuring tremors, the movement of a patient or measuring the pulse rate. Piezoelectric sensors work on the principle that they react to deformation by generating measurable electrical voltage. They can be used to measure force, flexion, extension and other values. The problem is that a piezoelectric sensor reacts very badly to low frequency changes such as respiratory frequency. We can find the use of these piezoelectric sensors for contactless monitoring of vital signs in U.S. Pat. No. 6,984,207, for example.
Capacitive sensors are often a part of medical equipment, and their advantage is that compared to piezoelectric sensors they are also sensitive to low frequency mechanical changes which are result of applied forces. For this reason they have a wide range of uses, from measuring the level of liquid, measuring position or measuring force, which can be used to measure a patient's respiration, for example. The use of these capacitive sensors is described for example in patent application WO 2006131855.
Contactless measurement of a patient's vital signs may be performed using inductive sensors that measure the bio-impedance of the patient, on the basis of which the patient's physiological expressions are evaluated. Such an embodiment is given, for example, in patent application WO 2006129212.
A modern trend in medicine is lower intervention in the patient's daily activities and so contactless measurement of the patient's vital signs is more attractive. Most often embodiment of the present measurement of vital signs is a mat consisting of one or more types of sensors. The sensors are for example piezoelectric, pressure or capacitive sensors. These sensors differ in terms of their ability to react to mechanical changes resulting from the patient's vital signs. For example a piezoelectric sensor is distinguished by the fact that it reacts well to dynamic changes which can be caused for example by the patient's pulse. Capacitive sensors react well to slow changes such as a patient's respiration. The problem given by making contactless equipment for the measurement of a patient's vital signs is that the sensors must be sensitive to even slight changes caused mainly by the breathing and pulse of the patient and they must not be disrupted by ambient forces. This can be achieved through a combination of different types of sensors but it leads to very expensive measuring devices.